This invention relates to a method for forming a substrate having high definition and high light-shielding capability. More particularly, it relates to a method for forming a substrate which has a light-shielding layer and which may be utilized for preparing a color filter employed in particular for a liquid crystal display device, and to a substrate having the light-shielding layer.
This invention also relates to a counterelectrode substrate for a thin film transistor (referred to as TFT) array substrate for black and white display which is superior in high definition and light-shielding capability and which may be prepared by a simple process, and to a liquid crystal display device for black and white display (LCD).
As a typical example of the color liquid crystal display device, there has hitherto been known a TFT (thin film transistor)--active matrix color liquid crystal device. The device includes a substrate arranged on an inner polarizing plate, a thin film transistor (TFT) and a pixel electrode driven by the TFT arranged on the substrate, and an inner alignment layer contacted with both the TFT and the pixel electrode. On its outer side, the device includes a liquid crystal layer having an outer alignment layer and an outer polarizing plate arranged on the outermost side. Between the outer alignment layer and the outer polarizing plate, there is mounted a color filter having a black light-shielding layer (black matrix) and colored layers, such as red-hued layer, green-hued layer and blue-hued layer on a transparent electrically conductive layer formed on the transparent substrate. For producing the color filter, there are currently proposed a dyeing method, a dye-pigment dispersion method, a printing method, an electrodeposition method and a transfer method. With any of these known methods, the precision in the arraying of the respective colored layers, such as red-hued, green-hued and blue-hued layers, herein occasionally abbreviated to R, G, B and BL layers, respectively, is of utmost importance. Above all, the black-hued layer, that is the light-shielding layer, needs to be positioned not only in registry with the counterelectrode substrate to avoid light leakage in the vicinity of the pixel electrode but also without voids between the other colored layers and the light-shielding layer significantly influences the picture quality such as contrast. Consequently, the current practice is to produce the light-shielding layer with high precision and to form other colored layers so as to be overlapped to some extent with the light-shielding layer. For example, an evaporated film of metal such as chromium is patterned using a photolithographic technique to produce a black matrix and the color layers of R, G and B layers are formed with small amounts of overlap with the black matrix at the boundary regions thereof.
With the thin film transistor (TFT) display device for black and white display, which has a construction similar to the above-mentioned color liquid crystal display device, a transparent substrate formed with a light-shielding layer (black matrix) is provided in place of the R, G, and B layers of a color filter, and functions as a counterelectrode substrate. In preparing the counterelectrode substrate, a resist is coated on a metal chromium layer formed by sputtering on a glass substrate, and a black matrix is formed by light exposure, development, etching and film exfoliation. A transparent ITO film is subsequently formed by sputtering on the entire surface.
However, if a light-shielding layer is formed of metal, manufacture-related problems are presented in that the evaporation method or lithography is susceptible to pinholes and involves a complicated process, and that the light-shielding film formed of metal has high light reflectance and leads to inferior viewing properties of the display device. Above all, with the TFT black and white display device, the vacuum process needs to be carried out twice in order to produce the black matrix and the electrode. If the method of overlapping the boundary regions of the colored layers is employed for the preparation of the color filter, it is not possible to produce a color filter having superior surface planarity which is strongly desired when the color filter is used for the color liquid crystal display device.
In order to overcome these problems, a method of employing a photosensitive resin composition admixed with pigments of black or the like dark or thick colors has been proposed in the Japanese Laid-Open Patent Publications Nos. 63-314501, 1-293306 and 5-34514. Specifically, a method of forming a photosensitive resin composition previously colored in a dark color on a transparent substrate, exposing via a pattern mask only the portions of the resin composition required as a light-shielding layer, for curing the resin composition and developing and removing only the unexposed portions of the resin composition, a method of forming a layer of a photosensitive resin composition previously colored to have a thick color on a substrate on which R, G and B layers have been formed, exposing the reverse substrate surface, that is the substrate surface not having the layer of the photosensitive resin composition, to light for curing the photosensitive resin composition and developing and removing only the unexposed portion, and a combination of these methods, are disclosed. However, the photosensitive resin composition colored to have a dark color hue exhibits high light absorption so that curing cannot proceed to a sufficient depth on exposure to light. Consequently, the photosensitive resin composition colored to have a thick hue tends to be removed during removal by development so that the light-shielding layer having a high light-shielding capability can hardly be produced. In addition, the photosensitive resin composition having its exposed portion cured by photopolymerization is frequently employed. In light exposure in atmospheric air, curing is obstructed significantly by oxygen contained in atmospheric air, such that complex preventative measures such as provision of an oxygen interrupting film or employing an atmosphere free of oxygen, such as vacuum or an argon atmosphere, are needed in carrying out the light exposure. Although it may be envisaged to eliminate such cumbersome operations by increasing the amount of light exposure to an extreme degree, reflection, scattering or leakage of light is increased, while the substrate temperature tends to be raised, thus presenting difficulties in the formation of the high-precision light-shielding layer.
On the other hand, if the photosensitive resin composition containing black-hued or nearly black-hued pigment is employed as a black matrix for a counterelectrode substrate for a TFT array substrate, and a transparent substrate having a transparent electrode is employed as a counterelectrode substrate, it is necessary to provide a transparent electrode by sputtering on the overall surface, because the black matrix itself lacks electrical conductivity. The reason is that, if the liquid crystal on the black matrix is not responsive to electrical voltage, the liquid crystal portion in the vicinity of pixels undergoes light leakage during voltage-on time with the normally white system employed in the TFT array system, thus lowering the contrast.